My son has a 2010 mustang gt 4.6L 5 sp, 3.73 diff gears, and the car makes a whining sound at speeds above 50, regardless of the gear its in and the noise gets louder the faster you go , really loud over 100, checked the gear oil in the diff, its clean and full, I know this is a truck sight but, a diff is a diff thought maybe one of you guys have heard a noise like this

 

Very common. Usually the noise will get louder and start to be heard at lower speeds. Sometimes you can figure out where the noise is by sound. A quick to build frequency sound points to the pinion bearing, as drive shaft speed is 3.73 times wheel speed in your case.

A slow noise can point to a wheel bearing or carrier bearing, where rotation is at wheel speed. You can sometimes determine which side a wheel bearing noise comes from by driving past an observer on one side of the car then the other. Another way is to drive past a wall an listen to see which side gives you the louder sound.

And then there is heat. A failing wheel bearing will make heat more quickly than a good one. After a short drive, feel the end of the axle shaft, or housing near the end where the bearings are. If one side is much warmer than the other, bingo.

None of this is an exact science.

Sometimes you get gear whine as well.

As long as you aren't talking major sound and heat, you can drive a whiner a loooong way. If it bugs you, get the check book out and find someone who is good enough at it that they can rebuild the axle in his sleep. Pay him a little extra to do yours while he is awake.